1. Field of the Invention
The present invention relates to a detecting device, and in particular, to a detecting device that detects a physical amount by using change in an electrical resistance value.
2. Description of the Related Art
There is a technique of detecting a physical amount by using a resistor (hereinafter called “thermistor”) whose electrical resistance value varies with respect to change in temperature. For example, when detecting temperature by using an NTC thermistor, as shown in FIG. 6 as an example, the electrical resistance value decreases with respect to rise in temperature, but the changes in the electrical resistance value with respect to temperature are not linear. Even when a thermistor other than an NTC thermistor is used (e.g., a PTC thermistor or a CTR thermistor), the changes in the electrical resistance value with respect to temperature still are not linear. Therefore, generally, a thermistor is connected in series to a voltage divider resistor whose electrical resistance value is 3.9 kΩ as shown as an example in FIG. 7, and due thereto, the changes in the electrical resistance value can be made to be linear by using the voltage division output such as shown as an example in FIG. 8.
However, at a thermistor, the changes in the electrical resistance value with respect to temperature are great, and therefore, even if the changes in the electrical resistance value are linearized by structuring a circuit such as shown in FIG. 7, the temperature range at which linearity is obtained is limited. In order to detect temperatures of a wide range, the voltage division resistance value must be switched by a switch in accordance with the temperature region. A circuit structure such as shown in FIG. 9 for example can be given as an example of a circuit structure for detecting temperatures of a wide range. In this case, a voltage divider resistor unit 102 and a switch unit 104 are connected to a thermistor 100. The voltage divider resistor unit 102 is structured to include a voltage divider resistor 102A whose electrical resistance value is 240 kΩ, a voltage divider resistor 102B whose electrical resistance value is 3.9 kΩ, and a voltage divider resistor 102C whose electrical resistance value is 0.36 kΩ. The switch unit 104 is structured to include switches 104A, 104B, 104C. In the circuit shown in FIG. 9, the other end of the thermistor 100, to whose one end power source voltage VDD is applied, is connected to respective one ends of the voltage divider resistor 102A that corresponds to low temperature region TL (e.g., −30° C.<TL≦75° C.), the voltage divider resistor 102B that corresponds to medium temperature region TM (e.g., 75° C.<TM≦180° C.), and the voltage divider resistor 102C that corresponds to high temperature region TH (e.g., 180° C.<TH≦350° C.). (Hereinafter, when there is no need to differentiate among the voltage divider resistors 102A, 102B, 102C, they are called the “voltage divider resistors 102”.) Further, the other end of the switch 104A, whose one end is grounded, is connected to the other end of the voltage divider resistor 102A, and the other end of the switch 104B, whose one end is grounded, is connected to the other end of the voltage divider resistor 102B, and the other end of the switch 104C, whose one end is grounded, is connected to the other end of the voltage divider resistor 102C. Moreover, among the switches 104A, 104B, 104C (hereinafter called the “switches 104” when there is no need to differentiate thereamong), by turning on only the switch 104 that corresponds to the temperature range to be detected, linearization of the electrical resistance values can be aimed for at each of the temperature regions that are the low temperature region TL, the medium temperature region TM and the high temperature region TH, as shown in FIG. 10 as an example.
The techniques disclosed in Japanese Patent Application Laid-Open (JP-A) No. 2001-201404, JP-A No. 7-55588, Japanese Utility Model Application Laid-Open (JP-U) No. 3-39129, and JP-A No. 2002-310807 for example are other techniques for aiming to linearize the electrical resistance values of a thermistor. All of these techniques as well vary the electrical resistance value of the thermistor per temperature region.
JP-A No. 2001-201404 discloses a technique of switching the current value that is supplied to a thermistor, for each of plural temperature regions detected at the thermistor.
Further, JP-A No. 7-55588 discloses the following technique: in a temperature detecting device that detects temperature by using an element whose electrical resistance value varies due to temperature, by switching switches on and off, selecting any of plural resistors that are respectively connected in series to the element per temperature region and that are connected to one another in parallel, and supplying current to the selected resistor.
JP-U No. 3-39129 discloses a temperature detecting circuit that uses the voltage division value of a temperature sensor and a pull-up resistor as sensor output, and that has a switching element that selectively switches plural pull-up resistors having different electrical resistance values, with respect to the temperature sensor.
JP-A No. 2002-310807 discloses a temperature detecting device having a switch for switching plural pull-up resistors that are connected between an electric circuit and a thermistor, and that have respectively different electrical resistance values, and that pull-up the output of the thermistor.
However, in the circuit structure shown in FIG. 9, the electrical resistance values of the switches 104 are added to the voltage division resistance values. Therefore, when the thermistor 100 and the voltage divider resistors 102 become several hundred Ω due to temperature, the electrical resistance values of the switches 104 cannot be ignored, and detection errors arise.
Further, a method has been thought of, in order to make the effect of the electric resistance values of the switches 104 small, using field effect transistors (FETs) for example as the switches 104, and lowering the electrical resistance values by widening the gate widths thereof. However, the constructed surface area increases.
Moreover, a method has been thought of measuring the voltage division output and the drops in the voltages of the switches 104, and calculating and eliminating the voltage errors due to the switches 104. However, means for computing is needed which leads to an increase in size of the form and an increase in costs. Further, because a large zero scale vicinity of the AD conversion error is AD-converted, it is difficult to decrease the detection errors.
Still further, none of the techniques disclosed in aforementioned JP-A No. 2001-201404, JP-A No. 7-55588, JP-U No. 3-39129, and JP-A No. 2002-310807 can suppress detection errors due to the electrical resistance values of the switching elements.
Note that cases of detecting physical amounts other than temperature (e.g., the electrostatic capacity of a capacitor, the amount of strain of a physical object, or the like) by using the electrical resistance value of a resistance element are similar to cases of detecting temperature.